Angolan President Jose Eduardo dos Santos has appointed his daughter as head of state oil firm Sonangol, AngolaNational State Radio (RNA) reported on Thursday, as the government aims to turn around the struggling company after low crude prices hit revenue.

Citing a presidential decree, RNA said Isabel dos Santos would head the company, after the president fired the entire Sonangol board and appointed a new one.

Angola said in April it would restructure Sonangol to "increase efficiency and profitability".

Sonangol said in February that debt owed to foreign oil companies had soared and it expected a very difficult year.

Angola imports around 6 million cubic metres of refined products a year, according to national statistics.

Angola, a member of OPEC, is currently Africa's largest oil producer because of militant attacks and other problems that have cut output in Nigeria.

Dos Santos, who has been in power since 1979 and is one of Africa's longest-ruling leaders, said in March he intended to step down as president in 2018 but gave no reason for his decision and did not name a preferred successor.

Dos Santos' mild, inscrutable public demeanour belies his tight control of Angola, a former Portuguese colony where he has overseen an oil-backed economic boom and the reconstruction of infrastructure devastated by a 27-year-long civil war that ended in 2002.

Critics accuse him of mismanaging Angola's oil wealth and making an elite, mainly his family and political allies, vastly rich in a country ranked amongst the world's most corrupt.

Oil sales account for more than 90 percent of Angola's foreign exchange earnings, making Sonangol the biggest source of state funding.